Not applicable.
Not applicable.
This invention relates to a method of clearing liquid from a vacuum purge system in a manner that protects the vacuum pump from becoming flooded with liquid. In another aspect it relates to apparatus including a vacuum pump capable of purging a vapor space also containing liquid without flooding the pump.
Vacuum pumps are often used to transfer fluids from one location to another and provision must be made to prevent any liquid present from entering the pump causing it to flood and shut down. Unless very expensive vacuum pumps are used, such flooding can also damage the pump, requiring repair or replacement. In the prior art a number of strategies have been used to protect a vacuum pump from becoming disabled in this way. In general, such measures provide some protective device such as a shut-off valve or a liquid trap capable of detecting the presence of liquid near the pump inlet and closing the line leading to the pump before liquid can reach it. The liquid is then drained away, either automatically or manually.
U.S. Pat. No. 3,612,089, Beguiristain (1971) describes an oral evacuator that would be used by a dentist. A suction pump or fan creates a vacuum that draws air, water and tooth chips from a patient""s mouth into a collection chamber that traps liquid and solids, preventing them from entering the suction source. A buoyant valve ball closes the chamber outlet at a high liquid level, thereby cutting off suction to the chamber. A drain valve in the chamber automatically opens to empty the chamber upon sensing a loss of vacuum.
U.S. Pat. No. 3,601,140, Hooper (1971) discloses a similar system that could be used for dispensing beverages in an automobile, using the engine""s induction manifold as a vacuum source. Liquid is drawn from a storage container by the application of a vacuum through a chamber that serves as a trap to prevent liquid from entering the engine manifold. A drain valve in the trap is closed by the application of a vacuum and opens automatically when the vacuum is terminated, such as by turning off the engine. An additional safety valve is provided for closing the vacuum line in case any liquid should pass from the trap into the suction line.
U.S. Pat. No. 3,827,452, Baumgarten (1974) describes an automatic shut-off valve that protects a vacuum pump from flooding by closing the passage leading to the pump. In case liquid floods the valve, a flotation needle member inside the valve is forced into the passage leading to the pump, blocking any liquid from the pump intake. The system also has one or more liquid traps that require manual replacing when they are full. The shut-off valve is a safety measure to protect against human error. This system was developed especially for use in medical facilities.
While the above described systems may be suitable for their respective purposes, they are not suitable for purging chemical vapors which may be toxic in nature or require either recovery or special treatment prior to disposal. For example, Hooper contemplates draining his liquid trap beneath the automobile whenever the engine is turned off. Baumgarten describes manual replacement of the traps when they become full, thereby exposing an operator to whatever liquid they contain. The liquid evacuated according to the system of Beguiristain is primarily water that could be drained into the city sewer.
In the chemical process industry it is frequently necessary to purge process equipment such as reactors and treatment chambers between batch operations, storage and transfer vessels after they have been emptied, and process lines through which chemicals have passed. Purging can be carried out by using an inert sweep gas, such as nitrogen, but this technique substantially increases the volume of gas and vapor that must be treated prior to disposal. The amount of purge gas can be reduced or eliminated by evacuation of the zones requiring purge, and vacuum pumps are suitable for this service. In such a system residual chemical vapors and liquid clinging to wall surfaces are withdrawn from the purge zones and any liquid present is vaporized under the subatmospheric pressure produced by the vacuum pump. For a number of reasons generally stemming from process upsets, equipment failure or human mistakes, excess chemical liquid can be present in the evacuation lines leading to the vacuum pump. The simple use of a liquid trap or safety valve to protect the pump, as in the references given above, is not adequate if one is to avoid exposing workers to hazardous chemicals, risking expensive downtime for the operation, or releasing toxic or noxious fumes into the environment.
A solution to the above-described problems within the chemical industry is sorely needed and it is to this need that the present invention is addressed.
According to the method of our invention, an improvement is provided in a vacuum purge fluid flow system in which vapors are withdrawn from a purged volume through a vacuum pump. The invention protects the vacuum pump from being flooded by liquid that can accompany these vapors by the following steps:
(a) Any liquid that would otherwise enter the pump is collected in a trap reservoir positioned in the purge system between whatever volume is being purged and the vacuum pump.
(b) The presence of liquid in the trap which might be drawn into the pump is detected, and
(c) in response to detecting such liquid, the conditions of pressure or temperature or both pressure and temperature within the trap reservoir are sufficiently altered so that liquid present in the trap is vaporized and passed through the vacuum pump as vapor.
This procedure can be carried out manually, but is preferably automated in order to reduce the possibility of human error. For example, the presence of liquid in the trap can be observed through a window by an operator who can then manually reduce the absolute pressure in the trap or increase the temperature in the trap by application of heat, or both, in order to vaporize the liquid observed. The vacuum pump continues to operate and chemical vapors are passed through the pump to whatever recovery or disposal procedures are chosen. The preferred manner of reducing the pressure in the trap is by restricting the flow of chemical to the trap while maintaining operation of the vacuum pump.
In a method of operating the invention which is particularly advantageous for purging chemicals in the fabrication of semiconductors, upon detection of a given quantity of liquid in the trap, flow into the trap is totally stopped until sufficient liquid has been vaporized and exhausted through the vacuum pump. At this point flow to the trap is resumed until liquid is again detected whereupon flow is again stopped, and so on, repeating this cycle until liquid no longer enters the trap with the purged vapors. In this manner chemical liquids in the purge line caused by a temporary process upset can be handled efficiently and safely removed from the system.
The apparatus of our invention can be used to practice the above described method and includes in operative relationship the following features in fluid communication with a confined volume from which chemical vapors are to be purged:
(a) a vacuum purge pump having an inlet and an outlet,
(b) a conduit for fluids connecting the volume to be purged with the inlet of the vacuum pump,
(c) a trapping vessel positioned to form a part of the conduit between the purge volume and the pump inlet, this trapping vessel having a configuration which causes liquid passing into it to be separated from vapor and collected in the vessel,
(d) a liquid detector positioned to detect a predetermined quantity of liquid in the trapping vessel,
(e) means for causing vaporization of liquid within the vessel by the reduction of absolute pressure in and/or the introduction of heat to the vessel, and
(f) a controller capable of developing a signal indicative of liquid in the vessel operatively connected to the detector (d) and the vaporization means (e) to stimulate vaporization of liquid in the vessel upon the detection of a quantity of liquid therein.
The preferred means for reducing the absolute pressure within the trapping vessel is a control valve in the conduit between the purge volume and the trap. This valve can be partially or fully closed to restrict passage of fluid into the trap while the vacuum pump continues to operate, thereby drawing down the pressure in the trap. The means for introducing heat to the trap can be used alone or in cooperation with the means for reducing pressure in the trap. Also, the means for reducing pressure in the trap can be used alone, without the addition of heat. One or the other of these elements must be present in the means of item (e) for vaporizing liquid in the trap, or both elements can be present and controlled to act in cooperation to produce the desired result.
This apparatus can also be adapted to practice the cyclic procedure in which the control valve is repeatedly closed and opened upon detecting the presence and absence, respectively, of predetermined quantities of liquid in the trap. The controller in this embodiment can be a programmable logic controller (PLC) adapted to carry out this cyclic operation.